libass: Bump version and change site

Google Code is a closing forge, libass has already made the move to
Github.

Since a new release was available a bump was also in order.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>